In recent years, as a structural member of an automobile, a thin high-tensile steel plate or a thin aluminum alloy plate has been more often used in order to reduce weight of a vehicle body. When a formed product is released from press dies after the high-tensile steel plate or the aluminum alloy plate, which is as described above, is subjected to press forming, then a residual stress generated in the formed product is released. Thereby a springback deformation occurs, which causes a deterioration of dimensional accuracy of the formed product. Therefore, some press forming methods in which the springback deformation is hard to occur have been proposed.
Among them, methods disclosed in Patent Literatures 1 and 2 have been heretofore present as measures against a springback deformation (for example, a camber back) that occurs in a formed product (for example, a U-shaped member or a hat-shaped member) having a U-shaped cross section having at least a top plate portion and side wall portions continuous with both sides thereof in a width direction, and having a bent portion bent in a plate thickness direction of the top plate portion. Here, the camber back is a phenomenon that, in a case of forming the above-mentioned formed product having the bent portion, the formed product, which is bent, returns slightly to an original shape thereof after being released.
Patent Literature 1 discloses a method, in which a difference between a curvature radius of a top plate portion of a hat-shaped member and a curvature radius of a flange portion of the hat-shaped member is made larger than a height of the side wall portion, whereby the residual stress of the formed product owing to the press forming is reduced, and the camber back that occurs in the formed product is suppressed.
Moreover, Patent Literature 2 discloses a method, in which the bent portion of the formed product is deformed with pressure in an anti-swelling direction in the course of the press forming of the formed product, whereby the residual stress of the formed product owing to the press forming is reduced, and the camber back that occurs in the formed product is suppressed.